The present disclosure relates to the electronics field. More specifically, this disclosure relates to electronic devices based on flip-chip technology.
Electronic devices may include one or more electronic components, each one implemented by a (monolithic) Integrated Circuit (IC) on a corresponding chip (for example, of silicon). In these electronic devices, the chips may be mounted on (chip) carriers, so as to protect the chips from mechanical stresses and to connect them electrically. Each chip is provided with terminals for accessing its integrated circuit (i.e., for exchanging signals and receiving a power supply input). The terminals are connected to corresponding electrical contacts of the carrier that implement any input/output function of the electronic device, for example, in the form of a grid of balls in an electronic device of the Ball Grid Array (BGA) type.
Particularly, in electronic devices of the flip-chip type (also known as Controlled Collapse Chip Connection, C4) the chip is flipped to have an active (front) surface with its terminals facing corresponding lands of the carrier. The lands are arranged in a grid on a surface of the carrier (without any other electrically conductive material therebetween to limit any risk of short circuits), and they are generally connected to the electrical contacts arranged on an opposite surface of the carrier (for example, by means of through via-holes, or simply vias). The terminals are then directly soldered to the lands (i.e., by joining them with the addition of a solder material that melts without melting the adjoining parts). A space between the chip and the carrier is filled with a filler of electrically insulating material (for example, particle filled epoxy resin), so as to improve their mechanical connection and compensate for different thermal expansion coefficients.
Generally, a reflow soldering technique may be used to solder the terminals to the lands. Briefly, solder bumps are deposited on the terminals of the chip using one of the various techniques available in the industry (for example, evaporation, electroplating, screen printing). The solder bumps are brought into contact with the corresponding lands that may also be pre-treated with solder deposition, so as to attach to them temporarily (due to a tacky nature of a solder paste or of an added soldering flux agent). The assembly is heated up to melt the solder material contained in the solder bumps. The assembly is then cooled, hardening the solder material to create a solder connection between each terminal and the corresponding land. The above-mentioned soldering operation requires the use of a solder mask (or solder resist). The solder mask restrains the solder material, thereby limiting its spreading during the soldering, when it is at the liquid state (so as to prevent undefined wetting of the solder material on possible exposed conductors of the carrier and to prevent the formation of any solder bridges creating unintended short circuits). For this purpose, the solder mask includes a layer of insulating material that is distributed on the whole carrier (so as to ensure the required insulation function), with windows that are opened therein for exposing the lands only (so as to allow their soldering).
Different connection techniques are also known in the art for connecting the terminals to the electrical contacts. For example, in a wire-bonding technique the chip is mounted with a (back) inactive surface thereof on the carrier (so as to have its terminals facing away from the carrier); with each terminal being connected to the corresponding land by means of a wire of electrically conductive material that is soldered with its ends thereto. In this case as well, a solder mask is provided on the carrier to prevent the formation of any solder bridge (and to protect any possible exposed conductors against oxidation).
Generally, chips may be subject to heating in operation, especially when they involve the consumption of relatively high power (for example, of the order of 0.01-1 kW). Moreover, the heating of a chip may be not uniform, since it may concentrate in areas with more power demanding portions of its integrated circuit (for example, cores of microprocessors). The heat produced by the chip should be transferred to the external environment for its dissipation, in order to ensure the correct operation of the electronic device.
However, in an electronic device of the flip-chip type the heat dissipation effectiveness into or through the carrier may be not completely satisfactory (on the contrary of the electronic devices based on the wire-bonding technique wherein the inactive surface of the chip is attached to the carrier so that the heat may be transferred from the bulk of the chip to the carrier). Indeed, in this case the connection of the chip to the carrier has poor thermal conductivity. Particularly the heat transfer mainly occurs through the solder connections that have quite limited size (e.g., of the order of 15-25% of the active surface for a fully populated grid with lands whose diameter is half the solder connection pitch); conversely, the majority of the active surface of the chip is thermally insulated from the carrier (especially for chips with peripheral terminals only), since two additional bad heat conductors, i.e., the filler and the solder mask, are interposed between them. Fillers of thermal conductive material might also be used to improve the heat dissipation in-between the solder connections; however, the solder mask on the carrier still represents a thermal barrier hindering the heat dissipation.
Several applications would benefit from an improved heat dissipation into or through the carrier, for example, if not enough room is available to attach bulky heatsinks to the inactive surface of the chip to dissipate the heat or if the dissipation of the heat from both sides of the chip would allow using less sophisticated, and then cost effective, thermal materials and heatspreaders/heatsinks to ensure the required heat dissipation characteristics.
Conversely, the poor heat dissipation effectiveness of the electronic devices may adversely affect their performance and reliability (for example, in smart-phone applications). Moreover, it may require the use of sophisticated, and then expensive thermal materials to enhance the heat transfer.